REPRINTED FROM VOL. XII, TRANSACTIONS OF AMERICAN CERAMIC SOCIETY. 
(Read at Pittsburgh Meeting, February, 1910.) 


THE BEHAVIOR OF FIRE BRICKS UNDER LOAD 
CONDITIONS AT A TEMPERATURE OF 1300°C. 


BY 


A. V. BLEININGER anv G. H. BROWN. 
sige OF ILLINOIS 


AMS 


ee ae SF a 


TO7 
wie 
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THE BEHAVIOR OF FIRE BRICKS UNDER LOAD 
CONDITIONS AT A TEMPERATURE OF 1300°C.! 


BY 


A. V. BLEININGER AND G. H. Brown, Pittsburg, Pa. 


PRELIMINARY STATEMENT. 


In this paper the writers are dealing with the definite 
problem of the load-carrying ability of fire bricks, and 
they make no claim that the test described by them will 
discriminate as to the general usefulness or value of a 
refractory. A material which might make a poor showing 
in this test might be useful for many purposes, where the 
streneth at the temperature employed is a minor consid- 
eration. 

In the study of the effect of heat upon clays and clay 
mixtures, it is necessary to keep in mind the fact that the 
phenomena of fusion are by no means confined to tem- 
peratures close to the so-called melting or softening point, 
but manifest themselves already at heats hundreds of de- 
grees below the stage of viscous fusion. In fact, we can 
say that clays and and clay mixtures do not possess a 
definite melting point. In testing a specimen of fire clay 
or of fire brick, fusion is said to have taken place when 
the test pyramid or specimen has softened sufficiently so 
that its edges are rounded, and it has bent over in the well 
known manner. Asa matter of fact, for all practical pur- 
poses, a fire brick would have been completely distorted, 
and would have shown all signs of fusion at considerably 
lower temperatures owing to its inability to support its 
own weight. It is simply due to the viscosity of the mass 
and to the fact that no load is imposed upon it, that it does 


1 By permission of the Director, U. S. Geological Survey. 


] 


2 BEHAVIOR OF FIRE BRICKS UNDER LOAD CONDITIONS. 


not show the distortion coincident with softening. It is 
evident, therefore, that the determination of the so-called 
melting point of a clay or fire brick, though useful in 
differentiating between low and high grade materials, 
does not offer a reliable means of expressing the entire 
refractory behavior. 

Thus Purdy’ found the fusion points of evidently in- 
ferior clays to be quite high. He reports, for instance, a 
clay of the formula 5.1 SiO,: Al,O, ° 0.046 Fe,O,- 0.09 TiO, 
to have a “fusion” point corresponding to cone 32. Simi- 
lar results have been obtained in the Survey laboratory at 
Pittsburg. 

The function of viscosity in extending the softening 
period is clearly recognized by all who have worked with 
silicates. 

Thus Day and Allen,? in their investigations, briny 
out this point very clearly. Doelter, and his students in 
fact, have endeavored to determine tlie viscosity of differ- 
ent silicates at various temperatures. Vogt, likewise, has 
sought to correlate the viscosity of fused magma with the 
sequence and velocity of crystallization. 

Greiner? determined, by means of a specially con- 
structed apparatus, the viscosity of mixtures of Na,Si0, 
with various other silicates at different temperatures. 
Thus, in a mixture of Na,O, SiO. and Al.O, he found that 
the viscosity was greatly increased by the addition of as 
small an amount as 2.5 per cent of Al,O,. This is shown 
eraphically in Fig. 1, where the abscissse represent the 
temperatures and the ordinates the relative viscosity. 
Curve I corresponds to the formula Al,O,°12 Na,O: 15 
SiO, and Curve II to Al,O,:9 Na,O:12 SiO,. If alumina 
thus increases the viscosity of Na,SiO,, which is one of 
the most mobile silicates, it is evident that the internal 

iT. State: Geol. Survey, Bull No: 4, p. los. 


2 The Isomorphism and Thermal Properties of the Feldspar, Carnegie 
Inst., 1905. 


’ Inang. Dissertation, Jena. 1907, p. 17. 


BEHAVIOR OF FIRE BRICKS UNDER LOAD CONDITIONS. 3 


TRANS. AM. CER. SOC. VOLXII. BLEININGER & BROWN. 


78509 Boe ~ 150° 12002 
Temperature m %. 
iriction of highly aluminous combinations must be very 
high indeed. 

On the other hand, the criticism raised as regards 
melting point determinations, does not apply to silicious 
mixtures containing a base-like lime. In testing silica 
brick it has been found that the melting point is well de- 
fined, as we should expect from our knowledge of the 
calcium silicates. Such a brick will stand up well in the 
fire, even under heavy loads, without deformation, at tem- 
peratures at a safe interval below its melting point. There 
is practically no flow until the melting point has been 
reached, when it will suddenly fuse to a liquid of low 
viscosity. This is probably the reason why silica brick 
are now being employed in many places where load con- 
ditions are an important factor, as in gas benches, ete. 

In order to overcome the viscosity effect of the fused 
mass it evidently seems desirable to bring about conditions 
which will neutralize it. For this reason some investiga- 
tors have found it advisable to apply a slight load to the 


4 BEHAVIOR OF FIRE BRICKS UNDER LOAD CONDITIONS. 


specimens to be examined. In some cases a platinum 
weight is used which is placed on top of the small cylinder 
or cone in the furnace. Howe’ has suggested a furnace 
illustrated in Fig. 2, which arrangement makes it possible 
to measure the rate of settling or flow as well. 


TRANS, AM.CER.SOC.YOL XI. BLEININGER Kk BROWN, 
U x 


y 
y 
y 
\ ) 


= 


For the solution of the problem at hand, viz., the re- 
sistance of fire bricks to load conditions, it was decided 
not to approach the softening temperature proper, at which 
the whole mass becomes a viscous liquid, but to restrict 
our work to average furnace temperatures at which only 
the more fusible silicates are softening. In the average 
fire brick, containing from 80 to 85% of flint clay and 


1 Metallurgical Laboratory Notes, p. 50. 


ELHAVIOR OF FIRE BRICKS UNDER LOAD CONDITIONS. 5 


20 to 15% plastic bond clay, it is obviously the character 
of the latter which would determine the behavior of the 
brick from this standpoint, since the flint clay within this 
region shows no fusing action whatever. In order to bring 
out the point sought, it was evidently necessary to apply 
loads somewhat greater than those used in practice, 
though it was realized that it must not be excessive. 


DESCRIPTION OF TEST. 


In preparing for this investigation the work of Lemon 
Parker? was consulted, whose interesting results were 


Ses 


TRANS. AM. CER. SOC. VOL Xl.  BLEININGER & BROWN 


See Shite, 
rer re 


2 Transactions American Ceram. Soe.. Vol. 7, Part II. 


6 BEHAVIOR OF FIRE BRICKS UNDER LOAD CONDITIONS, 


found very suggestive, though in his work only small 
specimens were used. 


Since it was thought advisable to use full sized bricks, 
a furnace was constructed as illustrated in Fig. 3, with an 
interior space of 20’’x 20’x 21”, heated by means of eight 
Fletcher burners, which were supplied with natural gas 
and compressed air. The combustion gases escape through 
holes along the back side of the furnace, which consists 
of a movable dour made from a heavy wrought iron frame 
lined with fire bricks. This frame is hung from two rollers 
running on an overhead track, so that it can be easily 
moved. The brick to be tested is firmly placed on end upon 
a heavy fire clay block about 9 inches from the door, and 
made to set plumb by means of fire clay mortar. The half 
of a chrome brick is then placed upon the test specimen 
and on top of it a long cylinder, made from high grade fire 
clay which extends through a hole in the arch of the fur- 
nace. The load is then applied upon a cast iron knife-edge 
plate by means of a steel I-beam 9 ft. long, with a distance 
of three feet between the pin taking the up-thrust and the 
fulcrum, and six feet between the latter and the point of 
load application. The weight of the beam is 155 pounds. 
The weight applied consists of a wooden box containing 
the necessary amount of iron shot. The beam was first 
calibrated by means of a platform scale. The pin at the 
one end of the beam passes through a cast-iron block slid- 
ing in a channel iron provided with a sht and a heavy 
fastening bolt. On top of the block two bolts are arranged 
so that it can be lowered by loosening the fastening bolt 
and tightening up the top nuts. In this manner the [-beam 
may be kept level throughout the test, thus avoiding any 
side strains upon the brick. 


Through an opening in the door a thermo couple is 
inserted, which is placed as close to the brick as possible. 
The junctions of the element are placed in test-tubes im- 
mersed in a beaker of water, the temperature of which is 
read regularly. 


BEHAVIOR OF FIRE BRICKS UNDER LOAD CONDITIONS. T 


Before beginning the regular tests it was, of course, 
necessary to determine the best working temperatures and 
pressures for the purpose in view, and in this the exper- 
ience of Mr. Parker, as given in the article cited above, 
was used as a guide. 

The first point it was necessary to determine was the 
rate at which a fire brick could be heated up, and the time 
required until the interior of the test specimen had as- 
sumed the temperature of the furnace. For this purpose 
a hole was drilled to the center of a brick. A thermocouple 
was then inserted and the hole well plugged up, leaving 
the couple well covered for some distance away from the 
brick. Another couple was placed close to the brick, but 
not touching it. Fig. 4 shows the time-temperature curves 


TRANS. AM. CER. SOC. VOL XII. BLEININGER & BROWN. 


1300 

oe Welle 
Bee bette dak [er 
roe Soh 


RE LEE 
Time /n Min. 
resulting from this test, which illustrate that after 285 
minutes, at the rate of heating followed, the inside tem- 
perature was the same as that on the outside. Since it was 
necessary to conduct the test with reasonable rapidity, ow- 
ing to the limited hours during which power was available, 


8 BEHAVIOR OF FIRE BRICKS UNDER LOAD CONDITIONS. 


the time-temperature curve shown in Fig. 5 was finally 
adopted. 

The temperature to which the brick was to be brought 
and held for one hour in one series of tests was taken to 
be 1800°C., based on the consideration that the more easily 
fusible combinations of silicate mixtures vitrify and soften 
within this heat region. In the second series the tempera- 
ture was raised to 1350°C. The tests described in this 


TRANS. AM. CER. SOC. VOL XII. BLEININGER & BROWN. 
1400 
00 
[200-—> 


ALU cares exec 


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Temperature (at 


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article refer to the former temperature. As regards the 
loads to be used, a considerable number of preliminary 
tests were made with pressures ranging from 125 pounds 
to 50 pounds per square inch, but in the first series a load 
of 75 pounds per square inch was employed, and in the 
second, 50 pounds. The present results refer to the 75 
pound load condition maintained for one hour at 1300°C. 

In addition to the load tests, the following determina- 
tions were made: 


we 


BEHAVIOR OF FIRE BRICKS UNDER LOAD CONDITIONS. 9 


en 


Crushing strength of the bricks on end in the 
cold condition. 

Chemical analysis. 

Softening temperature. 

Porosity. 

True specific gravity. 

Decrease in water absorption on re-burning 
to cone 12. 


D> OUR go bo 


1. The crushing strength of the bricks, placed in the 
machine endwise, was carried on in the usual way, pains 
having been taken to imbed the specimen in plaster. 

2. For the purpose of the chemical analysis, a sample 
sufficiently large was obtained by breaking up two half 
bricks and crushing them to pass the 100 mesh sieve. The 
disintegrated sample was allowed to flow in a small stream 
over a powerful magnet until all of the particles of metallic 
iron were removed. The analysis was carried on under the 
supervision of Mr. P. H. Bates, chemist in charge of the 
structural materials chemical laboratory, assisted by Mr. 
A. J. Phillips. The methods advocated by Hillebrand were 
used throughout this work. 

3. The softening temperature was obtained by chip- 
ping off specimens from the bricks and placing them in a 
Fletcher gas furnace, using preheated compressed air and 
natural gas. For the higher temperatures a small carbon 
resistance furnace was employed, in which the muffle con- 
sisted of a body containing 85% calcined alumina and 
15% of kaolin, Fig. 6. This furnace was found very satis- 
factory for the purpose. Seger-Orton cones were used for 
the determination of the temperatures. The results were 
checked in each case. 

4. The porosity was determined by obtaining the 
weight of a dry piece of the brick, the weight of the same 
specimen saturated with water by boiling in vacuo and the 
suspended weight. The results checked practically with 
the porosity, calculated from the true and apparent specific 
gravities. 


10 BEHAVIOR OF FIRE BRICKS UNDER LOAD CONDITIONS. 


TRANS.AM.CER.SOC.VOLXI. BLEININGER X BROWN. 


a Section A-A. 
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5. The true specific gravity was obtained from 50 
gram samples of the pulverized bricks, under the custo- 
mary precautions as to the removal of air by boiling in 
vacuo and making corrections for the final temperature 
of the water. : 

6. The original weighed bricks in this case were 
tested for water absorption by placing them flatwise in a 
covered tank, containing 114 inch of water, for 48 hours. 
After weighing they were dried and burnt in a down-draft 
test kiln to cone 12. They were then again immersed as 
before, and the water absorption determined. 

In this work 26 brands of fire bricks were tested, and 
each manufacturer was requested to furnish 20 bricks. 


BEHAVIOR OF FIRE BRICKS UNDER LOAD CONDITIONS. 11 


The writers desire to take this opportunity for thanking 
the various companies for their co-operation, and the in- 
terest taken in this work. 


RESULTS OF TESTS. 


In Table I the chemical analyses, as well as the calcu- 
lated empirical formula, are compiled. 

The results of the physical tests are arranged in Table 
II, and the data referring to the load test, as has been said 
above, apply to the load of 75 pounds per square inch and 
the temperature of 1300°C. 


BEHAVIOR OF FIRE BRICKS UNDER LOAD CONDITIONS. 


12 


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14 BEHAVIOR OF FIRE BRICKS UNDER LOAD CONDITIONS. 


'ANS.AM.GER.S0C VOL XIloge gages 


Fig. 7 represents the fracture resulting from the 
crushing test in the cold. The break is normal, and ayrees 
with the theoretical considerations. In the following plates 
the photographs of the fire bricks. after having been sub- 
jected to the load test, are reproduced. 


% 


BEHAVIOR OF FIRE BRICKS UNDER LOAD CONDITIONS. 


-——— a = — SS 


 TRANS.-AM. CER. SOC. VOLXI. ———BLEININGER & -BROWN 


= Figs Ht, 12,13. 


16 BEHAVIOR OF FIRE BRICKS UNDER LOAD CONDITIONS. 


BEHAVIOR OF FIRE BRICKS UNDER LOAD CONDITIONS. 


VOLKL. 


Figs 23,2 4, De, 


18 BEHAVIOR OF FIRE BRICKS UNDER LOAD CONDITIONS 


BEHAVIOR OF FIRE BRICKS UNDER LOAD CONDITIONS. 19 


“TRANS. AM. CER. S0C. VoL Xl. BLEININGER & BROWN.” 


Fig 3h 


Fig oo. 


ES 


Irom these it appears that in the cases where the 
bricks were badly distorted and crushed, in each case a 
certain degree of softening took place, as is clearly indi- 
cated by the curved surfaces. It is evident that these 
bricks attained a viscous condition in which they were not 
able to carry the load imposed upon them, though ‘the lat- 
ter 1s small compared with the crusting strength at the 
atmospheric temperature. In no case is the fracture as 
Sharp as is shown in Fig. 7, but invariably clear evidence 
of flow is produced. 

In carrying on the load test as the temperature rises, 
the beam is first raised, due to the expansion of the furnace 
bottom and the brick, a quiescent stage is then reached 
after which, from 1130 to 1290°, a weil defined deflection 
begins, caused by the contraction of the brick. In some 
bricks this deflection continues at a very slow rate, or 
reaches a condition of equilibrium some time after the 
temperature has been raised to 1300°. This kind does not 
fail under the conditions of the test, and the later deflec- 
tion starts the more apt is the brick to stand up. The 
materials failing under this test show a more or less early 


20 BEHAVIOR OF FIRE BRICKS UNDER LOAD CONDITIONS. 


g@, and the rate at which this takes place 
increases with the temperature, till finally it becomes so 
rapid that it is impossible to keep the beam level. [Failure 
then is merely a matter of minutes, and takes place very 
suddenly. In every case, therefore, softening precedes 
failure. With a third kind of material (of silicious com- 
position, low in fluxes) the rise in height remains station- 
ary for quite a while, followed by a long quiescent period. 
After the maximum temperature is reached a very slow 
and gradual settle takes place, which is quite small and 
decreases in rate as the temperature is kept constant. Such 
materials remain perfectly straight, and shrink but a small 
amount. 


Undoubtedly some of the bricks tested would have 
been deformed badly on long time tests, which it was im- 
possible to carry out under the conditions of the plant, but 
in several cases where the time of testing was extended 
considerably a kind of equilibrium seemed to be reached, 
after which no observable changes took place. It is very 
likely that in these cases solution of the more refractory 
portion of the brick took place, which stiffened up the 
bonding material. | 

Inspection of the failures shows plainly that the more 
refractory flint clay has not softened to the slightest ex- 
tent. The grains have lost none of their original identity. 
They seemed to have slid upon each other, the bond clay 
acting analogously to a lubricant. From this it follows 
that, no matter bow excellent the major constituent of the 
brick may be as to refractoriness, if the bond clay is defi- 
cient the load carrving power of the product is impaired. 
In other words, as far as this property is concerned, a fire 
brick is not any better than its weakest constituent. 


start in settling 


It is observed that there is some rough relation be- 
tween the softening temperature and the ability of the 
bricks to stand up under this test, but it is also seen that 
this is not true unqualifiedly. Test No. 16 is one of the 
decided exceptions. The failures not only show a softening 


BEHAVIOR OF FIRE BRICKS UNDER LOAD CONDITIONS. 21 


point below cone 30 but also a certain high content of 
fluxes, i. e., K,O0, Na,O, MgO, CaO and FeO. On the other 
hand, while a brick may soften below cone 30, if its content 
of RO constituents is low it will resist the conditions of 
the load test without difficulty. In No. 24. we have an 
illustration of this kind, and many silica brick show similar 
results. Nevertheless, it seems quite clear that the load 
test is in a measure a determination of the refractoriness, 
in spite of the low temperature at which it is conducted. 


TRANS. AM. CER. SOC. VOL XIl. BLEININGER & BROWN 


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No. 19 is not properly placed in this chart, as will be seen by reference 
to Table I.—Editor. 


In ig. 34 the failures due to the crushing of the 
bricks are plotted, and it is observed at once that the most 


22 BEHAVIOR OF FIRE BRICKS UNDER LOAD. CONDITIONS. 


significant factor seems to be the RO content. It is not to 
be expected that a sharp division line exists between the 
area of failure and that of satisfactory load carrving abil- 
ity, Since in some cases the FeO, which constitutes part of 
the RO, may be present in the form of larger grains, in 
which condition it is not detrimental. The fact is again 
shown that neither the softening temperature nor the 
chemical analysis alone produces the information brought 
out by the load test. Given all three of these data, how- 
ever, a very good presentation of the refractory resistance 
of a fire brick is obtained. 

Examining the question from the chemical standpoint, 
and assuming that the fire brick body consists of a refrac- 
tory constituent corresponding in composition to the kao- 
lin formula, and of a more fusible cementing component, 
the following method might be pursued, taking for example 
one of the failures, say sample No. 4. The entire brick, on 
calculation from the average analysis, possesses the em- 
pirical formula: 0.019 Na,O, 0.030 K,O, 0.036 MgO, 0.026 
CaO, 0.14 FeO, 0.054 TiO., 1.00 Al,O., 2.485 SiO... Assum- 
ing that the alkalies are derived from feldspar, the alumina 
corresponding to 0.019 -- 0.030 alkali is 0.049 equivalent. 
Deducting this from 1 we obtain 0.951 equivalent of clay 
substance, which contains 1.902 equivalent of silica. Sub- 
tracting this from 2.485 leaves 0.583 silica. Beside 9.951 
equivalent of pure clay base, this would leave the rest of 
the constituents to form the more fusible mixture: 0.019 
Na,O, 0.030 K,0O, 0.036 MgO, 0.026 CaO, 0.14 FeO, 0.049 
Al,O;, 0.583 SiO... Throwing this into another formula 
with RO equal to unity, we obtain: RO- 0.195 Al,O,° 
2.322 SiO. It is the amount and character (fusibility 
and viscosity) of this mixture which should govern the 
refractory behavior of the product under load conditions. 
I'rom a rough estimate, it would seein that this composi- 
tion is analogous to a quite fusible clay or acid slag, with 
comparatively low viscosity. The amount of this mixture 
might be calculated as follows: 


BEHAVIOR OL FIRE BRICKS UNDER LOAD CONDITIONS. 23 


0.049°37.35—= 1.83% AIO, 
0.583°54.58 = 12.83% SioO, 
.69% FeO 
.54% CaO 
.538% MgO 
.02% K,O 
.42% Na.O 


2.485 


oro C &8 


20.86% 


In this case, then, the fusible silicate mixture of the 
formulua RO-: 0.195 Al,O,° 2.322 SiO, would constitute 
20.86% of the total weight of the brick body, sufficient to 
cause it to behave as it did in the load test. In this 
theoretical speculation we must keep in mind that the 
effect is more marked, owing to the heterogeneous mixture 
of the assumed pure flint and impure bond clay. ‘The more 
thoroughly the two materials are ground and blended to- 
gether the better would the body behave in the load test, 
until finally the effect would be that corresponding to a 
single clay of the average composition, though even in this 
case it would remain a somewhat inferior material, owing 
to the high content of fluxes. The more impure the bond 
clay, therefore, the more complete and intimate should the 
mixture be. 

An analysis of the relation between the initial, cold- 
crushing strength and the load behavior, shows no appar- 
ent connection, but the fact is brought out that low inital 
streneth isa handicap. Bricks Nos. 4 and 11 are examples 
of this observation. While No. 4 would have failed irre- 
spective of its cold-crushing strength, the failure was more 
complete on account of its weakness, and No. 11 in all 
probability would have shown a very much smaller con- 
densation, in fact, would have stood the test. 

The hardness of burning in a general way is a factor 
worthy of consideration. Although burning to a high tem- 
perature cannot, in the nature of the case, effect any fun- 
damental change, and cannot convert a low grade material 
into a good one, our work has shown that well burnt bricks 
stand up hetter than soft burnt products. This is due, not 
only to the greater compactness of the hody, but also to 


24 BEHAVIOR OF FIRE BRICKS UNDER LOAD CONDITIONS. 


the change in the composition of the bonding material 
where such is used. In other words, hard burning will 
cause the plastic clay, usually decidedly less refractory 
than the flint, to dissolve some of the fine part of the better 


material, thus increasing its own refractoriness and hence 


its resistance to load conditions. For instance, No. 26 
would have shown up better if it had been burnt harder. 

To what extent other changes, such as the formation 
of sillimanite at higher temperatures, would have a bene- 
ficial effect, it was not attempted to determine. 


For the purpose of selecting materials for Government 


use, the tentative requirement was made that under the 
conditions of this test a 9” brick should not shorten more 
than one inch. 

The results of the second series of tests in which the 
load is 50 pounds per square inch and the temperature 
1350°C., are very much like those obtained in the first 
series, but appear to bring out more the question of flow 
due to softening being somewhat less dependent upon the 
initial strength of the brick. It is intended to use the lat- 
ter conditions in all further work. 


REFRACTORIES MADE FROM ONE CLAY. 


As to fire bricks made from a clay material which is 
sufficiently plastic so as to be used alone, it is clear that its 
ability to carry loads depends simply upon its composition. 
Some of the very best tests have been obtained from ma- 
terials of this kind. This case is the simplest and needs 
no particular attention, since a clay will stand up or fail 
by virtue of its own quality. A pure clay of moderate plas- 
ticity would be the ideal material for the manufacture of 
fire brick. Since, however, such clays are usually not avail- 
able, the question of judging them as to their composition 
deserves some attention from the standpoint of their load 
carrying capacity. From this point of view the most im- 
portant consideration is that of the amount of fluxes. 

At the high temperatures involved, the presence of 


Ap 


BEHAVIOR OF FIRE BRICKS UNDER LOAD CONDITIONS. 25 


even a sinall amount of fluxes becomes a potent factor, and 
hence it is far better to select a clay higher in silica and 
low in fluxes than a clay possessing the silica-alumina ratio 
of koalin but higher in fluxes. The effect of silica in lower- 
ing the refractoriness of clays is commonly exaggerated 
as far as practical results are concerned, and it is usually 
not necessary to reject a clay on this account. However, 
high silica and high fluxes make a dangerous combination. 
From this it follows that a clay fairly high in fluxes, corres- 
ponding for instance to 0.22 RO: Al,O, 2 SiO., would be 
improved by the dilution with a silicious material low in 
fluxes, or even by the addition of a clean sandstone, the 
principal requirement being intimate blending and erind- 
ing. 

In regard to the fluxes present in a clay, the state 
and size of grain of the iron is of importance, for it is 
evident that coarser grains of iron’ minerals will do no 
harm, though in the analysis they contribute their share 
towards raising the amount of fluxes. An effort should be 
made to determine the amount of such grains and to make 
the proper correction in the analysis. 

The fluxes are an important factor in the ability 
of refractories to carry loads, inasmuch as they become 
active at low temperatures, as has been shown in the table 
of results, by forming easily fusible silicates. As the tem- 
perature rises increasing amounts of silica and alumina 
are dissolved, and it becomes, therefore, simplv a matter of 
the amount of this fused matter and its viscosity, whether 
or not a given refractory will stand up at the temperature 
in question. As the temperature rises or the load is in- 
creased, a point is reached where the entire body becomes 
too viscous to retain its shape. 

By grinding the clay as coarse as possible, conditions 
are improved, as in doing this the formation temperature 
of the fusible silicates is raised, since the process of solu- 
tion is hindered, and by these means it may be possible to 
bring the product out of the danger zone. In time, however, 


26 BEHAVIOR OF FIRE BRICKS UNDER LOAD CONDITIONS. 


equilibrium conditious are approached closer and closer, 
so that finally the brick may fail under the same condi- 
tions under which they stood up at first. 


BRICKS MADE FROM FLINT AND BOND CLAY. 


In this case we are dealing with comparatively coarse 
grains of flint and fine grains of bond clay. ‘These two 
materials are mixed and blended as far as the processes 
customary at the present time permit it. The bond clay 
breaks up more readily, and the plastic mass produced by 
it cements together the grains of flint clay and caleine. 
Assuming that the flint clay is of good quality, the load 
‘carrying capacity of the product depends upon the bond, 
aS has been shown above. It is evident that if the flint is 
inferior the product suffers accordingly. These two ma- 
terials may be considered from the standpoints of chemical 
composition, vitrification range and fineness of grain. 


Flint Clay. 


This material, the geological origin of which is still 
in dispute, is deficient in plasticity, although by very fine 
erinding it may become sufficiently plastic to be molded. 
It is of conchoidal fracture, often quite hard, and at its 
best approaches pure kaolin in composition, being often 
surprisingly low in fluxes. Its drying shrinkage is prac- 
tically nil, but in burning it suffers as a rule a decided 
contraction Jn exterior volume. Its refractoriness, when 
pure, is very high. A brick made from flint clay (by fine 
and long continued grinding) is able to carry high loads. 
The vitrification range of flint clay has been well shown 
in the paper by Knote, in this volume. As far as refractor- 
iness is concerned, it is good practice to grind it quite 
coarsely for reasons already indicated. The fact that oe- 
casionally it is in part calcined does not affect its refrac- 
tory behavior, but simply corrects the shrinkage suffered 
in burning. : 

The physical characteristics of flint clay are not al- 


BEHAVIOR OF FIRE BRICKS UNDER LOAD CONDITIONS. ay 


ways a synonym for high refractory quality, as the writers 
have seen elays of this type which are decidedly inferior. 


Plastic Bond Clays. 

In these materials a good grade of plasticity is desir- 
able, though this is less important where their refractori- 
ness is high. In clays of greater fusibility high plasticity 
enables the manufacturer to cut down its amount to a 
minimum. In the selection of a bond clay it is evident 
that its vitrification range should be as long as possible, 
i. e., the temperature at which it becomes dense and non- 
absorbent should be as high as possible. The chemical 
composition and inherent mineral structure are thus most 
readily expressed by means of porosity-temperature curves, 
such as have been frequently described in these transac- 
tions, and which have been employed by Purdy in the 
study of Illinois fire clays. In making such tests, the time 
of burning should approach as much as possible the dura- 
tion of the burns in actual practice. What has been said 
of the chemical composition of clays under the head of 
bricks made from one clay applies also here. The main 
consideration is that the amount of fluxes be as low as 
possible, irrespective of the silica or alumina content. 

As.to fineness of grain, it might be said that the grind- 
ing should not be carried any farther than is necessary to 
develop good plasticity. 


Improvement of Bond Clay. 


If a flint-bond-clay mixture should prove unsatisfac- 
tory as regards its ability to carry loads, the first step 
would be to cut down the amount of plastic clay if possible. 
If this should not be possible or beneficial, the plastic ma- 
terial could be improved by grinding it intimately with a 
portion of the flint clay to such a degree of fineness as 
would insure thorough incorporation. This could be done 
by dry or, preferably, wet grinding. In the latter case 
it might be possible to pug the additional, coarser flint clay 
into the slip, which naturally would be maintained as thick 


28 BEHAVIOR OF FIRE BRICKS UNDER LOAD CONDITIONS. 


as possible. The finely ground flint would not remain 
inert but would be sure to possess some plasticity. 
Another alternative would, of course, be the sub- 


stitution of another clay for the unsatisfactory plastic 


bond. Since not infrequently silicious clays are at hand, 
this might prove feasible, even though the ainount of bond- 
ing material would have to be increased. Where the flint 
clay is becoming scarce, it might be possible to find some 
soft sand rock which could be ground into the plastic bond 
clay, and which would thus improve its behavior under load 
conditions. 

By some such means as have been indicated here, it 
would not seem a difficult matter in most cases to manu- 
facture products fulfilling all reasonable conditions as to 


crushing strength at the usual furnace temperatures. In 


addition, by paying attention to the sizing of the ground 
clays, So as to produce as dense a structure as possible, by 
the use of less water or by adopting dry pressing, the load 
carrying ability of the brick is increased. Likewise, burn- 
ing at a higher temperature assists in bringing about the 
same result. Experiments carried on by the writers have 
shown that harder burning has increased the streneth of 
fire bricks appreciably. 


CONCLUSIONS. 


1. The softening temperature of fire brick specimens 
is not a safe criterion of the resistance to load conditions 
at the average furnace temperatures. The same is true of 
the chemical analysis. By means of both the so-called 
melting point and the chemical analysts, the load behavior 
may be approximated. This cannot take the place, how- 
ever, of a direct test. The RO constituents (including the 
iron as FeO) should be as low as possible, and for best 
results should not exceed 0.22 equivalent. The content of 
fluxes is more important than the silica content. 

2. The test described appears to fulfill the require- 
ments of a load test for fire brick, since the results are 


t= 


te; 


BEHAVIOR OF FIRE BRICKS UNDER LOAD CONDITIONS. 29 


consistent and admit of close checking. It is recommended 
that the load be fifty pounds per square inch and the tem- 
perature 1350°C. A No. 1 brick should show no marked 
deformation and should not be shortened more than one 
inch, for the standard nine inch leneth. The test estimates 
in a measure the refractoriness. 

3. From the results of this test it appears that the 
amount and fusibility of the least refractory clay consti- 
tuent govern the behavior of the brick under load. Im- 
provement will result by grinding the less refractory clay 
intimately with flint clay or even silicious materials, low 
in fiux, improving the density, and by harder burning. 


DISCUSSION. 


Mr. Parker: I certainly take a great interest in the 
paper of Professor Bleininger, as I at one time undertook 
to make investigations of this very kind, but all were at 
higher temperatures.and with smaller loads. We were 
having trouble which was attributed to shrinkage, and I 
was soon satisfied that it was from some other cause. I 
would like to see these experiments carried a little further 
and at higher temperatures, which would result in data of 
very great value. 

Mr. Hipp: I would like to mention that recently we 
have done some work along this line, although our appli- 
ances were nothing like Professor Bleininger’s. I would 
like to ask whether Professor Bleininger thinks that the 
viscosity temperature, the temperature at which the bricks 
become viscous, has any direct bearing on the strength that 
the bricks might withstand? 

Mr. Bleininger: It appears that the viscosity of the 
brick at lower temperatures is the principal cause of 
failure. 

Mr. Hipp: Wo you think burning the brick harder in 
manufacture would increase the strength? 

Mr. Bletninger: J think so. It would tend to hold it 
up at higher temperatures. 


30 BEHAVIOR OF FIRE BRICKS UNDER LOAD CONDITIONS. 


Mr. Ramsay: As 1 understand Mr. Bleininger’s re- 
marks, the higher the crushing strength the better it will 
stand the heat. Take for illustration a fire clay, grind it 
very fine, and burn it ina kiln at a very high temperature. 
Will that brick stand more hot crushing strain than the 
brick that is not ground fine? 

Mr. Bleininger: Given two bricks of equal refractori- 
ness, the one with the greater initial compressive strength 
will have somewhat of an advantage. I*ine grinding does 


not necessarily mean a decrease in porosity, but rather the — 


reverse. The pores are smaller but the porosity is greater. 

Mr. Ramsay: With coarse ground material the dis- 
tance between the particles is greater than with fine 
ground. Under the heat conditions it takes longer to come 
in contact as they are further apart. 

Mr. Bleininger: We must remember that where flint 
clay is used in making fire bricks we are dealing with two 
materials, the flint clay and the bond clay. The latter is, 
as a rule, very much finer than the former. By a suitable 
combination of coarse and medium fine flint and fine bond 
clay, the greatest initial streneth is produced. It is evident 
that the more porous a brick is the less solid material it 
has to sustain the load. The coarser the clay is the longer 
it will resist the heat effect, but we must remember that 
in furnaces operated continuously we have a long time 
effect. It is obvious, however, that the bond clay cannot 
be coarse since this would defeat the purpose for which it 
is used. The weak point, therefore, is with the bond. If 
the latter is of high grade the brick will stand under the 
load, if not it will fail. But I have shown clearly how the 
product may be improved if the bond should not be of 
first quality. 

Mr. Ramsay: I wish to bring out, if possible, what 
relation the cold crushing strain will have to the hot crush- 
ing strain? 

Mr. Blewinger: There is very little relation. Ifa 
brick is physically weak under ordinary burning condl- 


eae 


BEHAVIOR OF FIRE BRICKS UNDER LOAD CONDITIONS. dl 


tions and is defective in composition, you can improve that 
material somewhat by making it. stronger, but it will not 
be a fundamental remedy. 

Mr. Ramsay: In other words, it one brick is soft 
burned and one is hard burned, the hard brick will stand 
the greater heat? 

Mr. Bleininger: WHard burnt fire bricks seem to stand 
up better than soft burnt. 

Mr. Hamilton: In Professor Bleininger’s investiga- 
tion he has found that the less refractory brick stood the 
greater test when cold? 

Mr. Bleininger: The cold strength, as I have said 
before, has direct relation to the strength in the furnace. 
We have tested highly refractory materials that showed 
a good strength when cold. 

Mr. Hamilton: And while the photographs do not 
show up as badly in the hot tests, that is the less refrac- 
tory brick. Now, we make a brick that we think is very 
highly refractory. That would be a rough grained brick 
containing a binder of 20% of plastic clay. That brick 
would be very porous and I take it that the hot strain 
would be very low, but at the same time that brick would 
stand a greater heat than 1300 degrees where it did not 
come in contact with a load. A brick containing flint and 
bonded with 30% plastic clay would not show up as well, 
but would bea satisfactory brick for the roof of a furnace. 
The thing that I would like to find out is, would it be 
better to make a fine ground brick than one coarse ground ? 
Your experiment has not shown that. 

Mr. Bleininger: I want to say that I think I have 
been misunderstood. By grinding the whole brick mixture 
finer you of course improve the standing up quality of an 
inferior bond clay. But it is not necessary to do this. If 
for instance you were to grind the bond clay with some of 
the flint very fine, and then mixed this combination with 
your coarser ground flint, as usual, you would obtain the 
best result. I say distinctly that I am speaking of brick, 


32 BEHAVIOR OF FIRE BRICKS UNDER LOAD CONDITIONS. 


suitable for carrying loads at furnace temperatures, and 
realize fully that a brick may stand up at a high tempera- 
ture if no load is imposed, while it would fail in this test. 
There is a very rough connection between the ability of a 
brick to stand up under this test and its so-called refrac- 
toriness. <A brick softening below cone 30 would not stand 
much of a chance in the load test. 

Mr. Parker: I think if Professor Bleininger goes a 
little farther in his experiments he will find that the re- 
sults do not agree at higher temperatures. Beyond 1300 
degrees C. they would not agree at all. At least that is 
my experience. 

Mr. Bleininger: Weselected the temperature because 
it is a very common furnace temperature. 

Mr. Parker: JI was compelled to go higher, and I 
found that one kind of clay standing unusually high tem- 
peratures was not as good a load carrier at a minimum 
temperature of 2400° to 2500° T°. as a less refractory clay. 

Mr. Hamilton: The point is, | do not think we can 
get a highly refractory brick sample and at the same time 
a brick that will stand up well under load conditions, be- 
cause you cannot introduce the bond and hold up with the 
temperature. 

- Mr. Yates: At higher temperatures in the making of 
fire brick you take all the shrinkage out of your clay. 

Mr. Hipp: I would like to say concerning the per- 
centage of plastic clay used, that we made some tests 
recently, trying different amounts, and found that two 
bricks of practically the same refractoriness, one contain- 
ing 30% of bonding clay and the other 50%, showed con- 
siderable difference as to their ability for withstanding 
the strain imposed. I would like to ask Professor Blein- 
inger whether in testing two fire bricks, each containing 
a different amount of plastic clay but the porosity about 
the same in each brick, the crushing strength would vary 
to anv great degree? 


BEHAVIOR OF FIRE BRICKS UNDER LOAD CONDITIONS. 33 


Mr. Bleininger: IJ cannot answer this question satis- 
factorily. 

Mr. Ramsay: I think you have stopped too soon. We 
know your position in this line of work, and our customers 
would be apt to take your opinion rather than ours, and IL 
think sometimes your conclusions are erroneous. I think 
that sometimes the clay used in bond has some effect upon 
the brick. 

Mr. Blemminger: Of course, we stand back of our re- 
sults, as they have been made with care, and I cannot re- 
tract one word of what I have said. When Mr. Ramsay 
speaks of the effect of the bond clay, [ wish to call atten- 
tion to the fact that this is the very thing which we have 
emphasized. 

Mr. Parker: I would like to say that at higher tem- 
peratures Mr. Bleininger will find different results. At his 
temperature they agree with my experiments. You will 
find at higher tempertures that an ordinary fire clay prom- 
ising good results as a load carrier, though still far from 
its fluxing point, may lose its efficiency in this respect, 
while the more refractory clay will prove more efficient 
with a moderate load to a temperature much nearer its 
fusion point. 


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